Wastewater collection systems are a critical part of urban infrastructure. They collect sewage from homes, businesses and industries and convey it to treatment plants before it's safely released back into the environment. These systems are complex networks of gravity sewers, holding tanks, pumping stations and pressurized pipes known as force mains. Gravity sewers are the primary means of collecting wastewater and conveying it to treatment plants. However, where excavation conditions are difficult or in flat areas or when wastewater needs to be conveyed across rivers or lakes, gravity sewers are not practical and wastewater must be pumped through force mains. Typically, in such situations the gravity sewers flow into holding tanks, from where the wastewater is pumped to gravity sewers on the other side of the river. Pumping is periodic, its duration and period depend on the rate of wastewater flow and capacity of holding tanks. Typically, the duration is between 3 to 5 minutes.
Gravity sewers and force mains deteriorate naturally with time and eventually lose their initial wastewater tightness, starting to leak. Deterioration is caused by corrosion, soil movement, poor construction standards, and in the case of force mains by repeated pressurizing and depressurizing. Leakage of wastewater is especially of concern in the case of force mains at river and lake crossings because it may go undetected for long periods of time and can have severe impact on the environment. A number of catastrophic incidents have occurred in Canada and the United States in recent years. This risk needs to be addressed and therefore there is an urgent need for reliable technologies to continuously monitor leakage in these critical pipes.
Technologies that may be applicable include acoustic leak noise correlation, mass balance, pressure analysis, and temperature monitoring using fibre-optic sensors.
Mass balance, pressure analysis, and temperature monitoring using fibre-optic sensors are costly to implement. Also, these technologies have been developed primarily for monitoring leakage in oil and gas pipelines under steady state conditions. They have not been demonstrated or evaluated for monitoring of wastewater force mains, which normally operate under transient conditions.
Acoustic leak noise correlation technology is well established for detecting and pinpointing leaks in water transmission and distribution pipes. However, like other technologies, its application to wastewater force mains had not been demonstrated in the past. It is commonly believed that the application of acoustic correlation to force mains is fraught with difficulties due to high background noise caused by nearby pumping stations, excessive signal attenuation caused by the presence of undissolved gases, and compressible solid matter; variable acoustic propagation velocity; relatively low pipe pressure; and the requirement for large sensor-to-sensor spacing.
Additionally, while leaks in pressurized water distribution systems can generally be detected using acoustic leak noise correlation technology, problems can arise when the leaks are very small since the generated noise level in this case can be very low.